Re: [PATCH 06/12] cachefiles: add consistency check for copen/cread

[Baokun Li <libaokun@xxxxxxxxxxxxxxx>]

Hi Jingbo,

Thank you very much for the review!

On 2024/5/6 10:31, Jingbo Xu wrote:
> Hi Baokun,
>
> Thanks for improving on this!
>
> On 4/24/24 11:39 AM, libaokun@xxxxxxxxxxxxxxx wrote:
> > From: Baokun Li <libaokun1@xxxxxxxxxx>
> >
> > This prevents malicious processes from completing random copen/cread
> > requests and crashing the system. Added checks are listed below:
> >
> >    * Generic, copen can only complete open requests, and cread can only
> >      complete read requests.
> >    * For copen, ondemand_id must not be 0, because this indicates that the
> >      request has not been read by the daemon.
> >    * For cread, the object corresponding to fd and req should be the same.
> >
> > Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> > ---
> >   fs/cachefiles/ondemand.c | 27 ++++++++++++++++++++-------
> >   1 file changed, 20 insertions(+), 7 deletions(-)
> >
> > diff --git a/fs/cachefiles/ondemand.c b/fs/cachefiles/ondemand.c
> > index bb94ef6a6f61..898fab68332b 100644
> > --- a/fs/cachefiles/ondemand.c
> > +++ b/fs/cachefiles/ondemand.c
> > @@ -82,12 +82,12 @@ static loff_t cachefiles_ondemand_fd_llseek(struct file *filp, loff_t pos,
> >   }
> >   
> >   static long cachefiles_ondemand_fd_ioctl(struct file *filp, unsigned int ioctl,
> > -					 unsigned long arg)
> > +					 unsigned long id)
> >   {
> >   	struct cachefiles_object *object = filp->private_data;
> >   	struct cachefiles_cache *cache = object->volume->cache;
> >   	struct cachefiles_req *req;
> > -	unsigned long id;
> > +	XA_STATE(xas, &cache->reqs, id);
> >   
> >   	if (ioctl != CACHEFILES_IOC_READ_COMPLETE)
> >   		return -EINVAL;
> > @@ -95,10 +95,15 @@ static long cachefiles_ondemand_fd_ioctl(struct file *filp, unsigned int ioctl,
> >   	if (!test_bit(CACHEFILES_ONDEMAND_MODE, &cache->flags))
> >   		return -EOPNOTSUPP;
> >   
> > -	id = arg;
> > -	req = xa_erase(&cache->reqs, id);
> > -	if (!req)
> > +	xa_lock(&cache->reqs);
> > +	req = xas_load(&xas);
> > +	if (!req || req->msg.opcode != CACHEFILES_OP_READ ||
> > +	    req->object != object) {
> > +		xa_unlock(&cache->reqs);
> >   		return -EINVAL;
> > +	}
> > +	xas_store(&xas, NULL);
> > +	xa_unlock(&cache->reqs);
> >   
> >   	trace_cachefiles_ondemand_cread(object, id);
> >   	complete(&req->done);
> > @@ -126,6 +131,7 @@ int cachefiles_ondemand_copen(struct cachefiles_cache *cache, char *args)
> >   	unsigned long id;
> >   	long size;
> >   	int ret;
> > +	XA_STATE(xas, &cache->reqs, 0);
> >   
> >   	if (!test_bit(CACHEFILES_ONDEMAND_MODE, &cache->flags))
> >   		return -EOPNOTSUPP;
> > @@ -149,9 +155,16 @@ int cachefiles_ondemand_copen(struct cachefiles_cache *cache, char *args)
> >   	if (ret)
> >   		return ret;
> >   
> > -	req = xa_erase(&cache->reqs, id);
> > -	if (!req)
> > +	xa_lock(&cache->reqs);
> > +	xas.xa_index = id;
> > +	req = xas_load(&xas);
> > +	if (!req || req->msg.opcode != CACHEFILES_OP_OPEN ||
> > +	    !req->object->ondemand->ondemand_id) {
> > +		xa_unlock(&cache->reqs);
> >   		return -EINVAL;
> > +	}
> > +	xas_store(&xas, NULL);
> > +	xa_unlock(&cache->reqs);
> >   
> >   	/* fail OPEN request if copen format is invalid */
> >   	ret = kstrtol(psize, 0, &size);
> The code looks good to me, but I still have some questions.
>
> First, what's the worst consequence if the daemon misbehaves like
> completing random copen/cread requests? I mean, does that affect other
> processes on the system besides the direct users of the ondemand mode,
> e.g. will the misbehavior cause system crash?
This can lead to system crashes, which can lead to a lot of problems.
For example, on reopen, to finish the read request, we might UAF in
ondemand_object_worker();
Or we might UAF in cachefiles_ondemand_daemon_read() when we
haven't added reference counts to the req yet.
Even though these issues are completely resolved in other ways,
I think some basic consistency checks are still necessary.
> Besides, it seems that the above security improvement is only "best
> effort".  It can not completely prevent a malicious misbehaved daemon
> from completing random copen/cread requests, right?
Yes, this doesn't solve the problem completely, we still can't
distinguish between the following cases:

1) different read reqs of the same object reusing the req id.
2) open reqs of different objects.

Ideally, we would calculate a checksum from
timestamps + struct cachefiles_msg to check if the requests
are consistent, but this breaks the uapi.

Thanks,
Baokun